This invention is directed to optoelectronic broadband switching arrays and in particular to a novel optoelectronic cross-point switch for use in such an array.
Linearity and power limitations of solid state electro-optic devices restrict to about three the number of frequency-division multiplexed analogue video channels that can be carried over reasonable distances via an optical-fiber link. Centrally switched distribution is thus the best method for providing a broadband communications service to subscribers on optical fibers. Since some subscribers may want simultaneous access to more than one channel, the central switching apparatus must be capable of selecting two or three channels and multiplexing them in a format compatible with the limited frequency-division multiplexing capability of the optical-fiber link to the subscriber. Either the switch must be capable of handling signals previously modulated onto their frequency-division multiplexing carriers, or each subscriber's connection at the central switching apparatus must be supplied with a set of modulators so that the switching can be done at baseband and then upconverted by the modulator to a frequency in the multiplexing scheme. The former approach uses far fewer modulators, and therefore considerable saving can be realised by a broadband switching array that is capable of switching VHF and higher frequencies. One optoelectronic cross-point switching array is described in U.S. Pat. No. 4,074,142 which issued to Albert S. Jackson on Feb. 14, 1978. This type of switching array though desireable may suffer from poor isolation between input and output channels at the cross-point switches particularly for high frequencies because it employs electronic switches separate from the photodetecting element, and thus must switch electrical signals.